An organic light-emitting device includes a thin film containing a light-emitting organic compound, the thin film being arranged between an anode and a cathode. Excitons of the light-emitting compound are formed by injecting electrons and holes from the electrodes. When the excitons return to the ground state, the organic light-emitting device emits light.
Recently, organic light-emitting devices have greatly improved. The characteristic features of organic light-emitting devices include high luminance at a low applied voltage, a variety of emission wavelengths, rapid response, and the fact that thin, lightweight light-emitting devices can be produced. Thus, organic light-emitting devices can be potentially used for a wide variety of applications.
However, there is still room for improvement in the present circumstances. Specifically, in order to achieve practical use, organic light-emitting devices are required to have a higher-intensity optical output or higher photoconversion efficiency. Furthermore, it is necessary to minimize the variation with time by prolonged use and to improve durability, for example, the resistance to degradation due to an oxygen-containing atmosphere, humidity, and so forth.
A technique for adding a small quantity of light-emitting molecules (dopant) to an organic light-emitting layer mainly composed of a host has been known. This technique is very important in the improvement of the luminous efficiency and emission lifetime of an organic light-emitting device. Various improvements have been made.
Furthermore, a technique for incorporating a dopant that functions of transporting carriers or transferring excitation energy in addition to a dopant serving as light-emitting molecules into a light-emitting layer has been known as the related art.
Among these techniques, a technique for confining charges in a light-emitting layer by trapping carriers injected in the layer by a dopant has been reported. For example, an organic light-emitting device including a luminous layer that contains a hole-trapping material and an electron-trapping material is disclosed (see PTL 1).
For the organic light-emitting device disclosed in PTL 1, holes and electrons are accumulated in the hole-trapping material and the electron-trapping material, respectively, contained in the light-emitting layer. Thereby, the hole-trapping material or the electron-trapping material emits light. With respect to the materials in which charges are accumulated, however, the occurrence of quenching due to the reaction of accumulated charges with excitons during the emission of light from the materials can result in the formation of charges that do not contribute to light emission. Furthermore, the hole-trapping material disclosed in PTL 1 contains a nitrogen atom and thus does not easily serve as a charge transporter for electrons in the light-emitting layer. As a result, the carrier balance tends to be poor. In addition, the hole-trapping material disclosed in PTL 1 easily forms a charge-transfer complex, possibly causing reductions in luminous efficiency and emission lifetime. There is a possibility that even if the hole-trapping material disclosed in PTL 1 is used in a low concentration in order to inhibit the formation of the charge-transfer complex, insufficient hole-trapping properties and hole injection properties are obtained.